You'll lose any rights to alternate methods of compensation and they will pocket most of the dosh.
2/8 to 20/8 does not seem make 5 weeks.
Your posting gives no detail of the manner which you used to report the fault. Who you bought it from, date of purchase, type of Warranty.
Assuming you jumped through all the hoops in the proscribed manner. Got a Returns Number or whatever form of record that Acer use. Packed it suitably. How did you pack it? et cetera, et cetera.
Then 18 days is not excessive, particularly considering that this is the holiday season.
I know I'm not being very sympathetic but unless a Company has a suitable way of tracking the mail, is is easy for an unidentified item to be side lined.